It is known that secure documents or instruments may be rendered less susceptible to forgery or counterfeiting by including security features in various forms within the body of the document. In fact, the security or integrity of a document or instrument will increase with the number of separate and distinct security features that it employs.
Many security papers and other items of value include a security device or element, such as a security thread, disposed on or within the document. The security device typically includes one or more security features, such as metallic, magnetic and/or luminescent security features that serve to authenticate the security paper and prevent or deter counterfeiting.
A common type of security thread includes metal-formed characters or indicia disposed on a plastic carrier substrate. Such threads, which are coated with a very thin (e.g., 300 to 500 angstrom) layer of metal, such as aluminum, and are then demetallized, display either: discrete metal characters as currently used in United States currency; negative or reverse-image characters as currently used in the new Euro currency; or a repeating pattern of isolated metal blocks containing negative or reverse-image characters as described in U.S. Pat. No. 5,486,022 and as used in Indian and Venezuelan currencies. These threads are visually detectable in transmitted light by members of the public and may be machine detectable by conventional thread detectors that detect the presence or absence of conductive features on the threads. The repeating patterns of the threads described in U.S. Pat. No. 5,486,022 and employed in Indian and Venezuelan currencies are machine readable as well as machine detectable.
Demetallized threads have well known advantages over printed security threads. Due to the presence of a reflective metal layer, beneficial optical effects are produced under circumstances where the thread is entirely buried in paper as well as when partially exposed as in the case of windowed thread. There are further advantages to the metallic layer when security threads are authenticated during banknote sorting. The presence of either a continuous metal layer or a specifically encoded patterned metal layer can be detected on the basis of conductivity.
A known disadvantage of demetallized threads is their susceptibility to corrosive effects during paper manufacturing and in circulating banknotes. More specifically, any metal extending to the edge of the thread, even when protected by a laminate or transparent coating, acts as a wick for chemicals that cause corrosion or degradation of the metal layer. As a consequence threads in banknotes exposed to even weak alkali solutions such as used in common household laundries may lose all or part of their metal layer via corrosion from the edges inward.
A need therefore exists for a security device that is resistant to chemical attack.
It is therefore an object of the present invention to provide such a durable security device.
It is a more particular object to provide a security device for use in security articles such as bank notes and the like, that matches or exceeds the durability of the security article.
It is another object of the present invention to provide a security article having at least one durable security device at least partially embedded therein and/or mounted thereon.